Transgenic mice offer the opportunity to explore the influences of gene function on the regulation of growth and differentiation in specific primary cell-types in the context of their natural environment. The epithelium of the large and small intestines contains four terminally differentiated cell-types, all of which are derived from a common multipotent progenitor. The multipotent stem cells reside near the base of the intestinal crypts and are continuously proliferating. As these cells migrate up the villus axis towards the lumen, they exit the cell cycle and differentiate into enterocytes, goblet cells, enteroendocrine cells or Paneth cells. In transgenic mice, the intestinal fatty acid binding protein promoter (I-FABP) directs the expression of genes under its control to the differentiated enterocytes. Expression of T antigen in these cells causes them to reenter the cell-cycle, resulting in a dramatic hyperplasia of the intestine. Our preliminary studies indicate that the amino-terminal 121 residues of T antigen is sufficient to induce intestinal hyperplasia. However, whereas wild-type T antigen will cooperate with the ras oncogene to induce dysplasia, the amino-terminal fragment does not. These results suggest that the ability of T antigen to physically associate with p53 is not required for hyperplasia, but may play a role in progression to dysplasia. We propose to examine the influence of individual T antigen transforming activities on p53 functions in intestinal enterocytes. Transgenic mice that express mutant T antigens defective in one or more transforming activity will be generated. The ability of these mutants to induce hyperplasia, cooperate with ras to induce dysplasia, and to alter specific p53 activities will be evaluated. These studies should allow us to: (1) determine which T antigen activities are required for hyperplasia and dysplasia; (2) whether the ability of T antigen to alter p53 function by a mechanism independent of complex formation contributes to hyperplasia and/or dysplasia; and (3) what specific aspects of p53 activity (gain or loss of function) contribute to intestinal neoplasia.